This invention relates to a condensed steam agitator for use in paper machine dryer cylinders and to a related method for installing a particular preferred form of the agitator in the dryer cylinders. The use of the invention, as particularly described hereinbelow, overcomes the many and various shortcomings associated with conventional means for reducing the substantial limitations on dryer efficiency presented by the condition known as condensed steam rimming.
In the papermaking field, an aqueous suspension of cellulose fibers or pulp furnish is typically passed through a moving wire as on a Fourdrinier machine, for example, where the rapid drainage of the water through the interstices in the wire forms the paper web. Thereafter the formed paper web may be passed through a press section whereupon it is typically conveyed through the dryer section of the paper machine where the moisture content in the sheet is reduced to its final levels. The dryer section typically utilizes a plurality of steam-heated dryer cylinders each of which is equipped with a steam inlet for introducing live steam into the interior of the cylinder and a syphon for removing accumulated condensate from the interior of the dryer.
At relatively low operating speeds, the syphon adequately performs its function of condensate removal. At the prevailing operating speeds of above approximately 1,000 feet per minute, however, the condensed steam tends to form a layer along the interior surface of the cylinder. This phenomenon, which is termed condensate rimming, impedes the heat transfer efficiency of the dryers since the layer acts as an effective insulator. And, since the industry is highly capital intensive, the limitations placed on dryer capacity by condensate rimming is highly significant both from the standpoint of product quality and production rates.
In order to reduce the detrimental effects of the rimming phenomenon, the use of a plurality of rails or spoiler bars to disrupt the condensate layer and thereby improve heat transfer through the dryer shell was proposed by Barnscheidt et al in U.S. Pat. No. 3,217,426. U.S. Pat. No. 3,724,094 to Appel et al disclosed a relationship between the optimum spacing of the spoiler bars on the one hand and the diameter of the respective dryer and depth of the condensate layer on the other. The use of spoiler bars presents its own set of limitations, however, including extended installation periods which require upwards of 10 man-hours per dryer primarily as a result of the need for careful spacing of the spoiler bars relative to each other. Moreover, spoiler bars are installed based on the estimated or average operational conditions for the dryers which obviously vary on a particular machine as production rates change and different paper grades or basis weights are formed on the same machine. Consequently, an otherwise effective arrangement at the time of installation can be rendered ineffectual almost immediately after installation by routine changes in operating conditions. And, while U.S. Pat. No. 4,195,417 to Mathews advocates the use of magnetic bars and suggests that the relative spacing of the bars may be changed in response to changing operational conditions, this is virtually impossible in practice where around the clock operation is the norm rather than the exception. In order to reposition the bars, a paper machine would have to be shut down for a sufficient time to allow the dryers to cool to room temperature since physical entry to accomplish the adjustment is otherwise impossible. Those familiar with prevailing realities in the papermaking industry immediately recognize the impracticality of undertaking such a procedure each time the operating conditions of the paper machine are altered. Notwithstanding the substantial limitations associated with the use of spoiler bars, they remain the predominant form of agitator employed in the industry as indicated for example, by the recent issuance of U.S. Pat. No. 4,542,593 to Vittanen et al for a spoiler bar arrangement utilizing an arrangement of attachment bands.
The Vittanen et al patent as well as Matthews recognize a further problem with the use of spoiler bars, namely that the use of fasteners which penetrate the dryer wall weakens the dryer shell. Although penetration of the dryer wall is avoided by Matthews' magnetic bars, these magnetic bars are very expensive, are susceptible to corrosion, and as a whole are sensitive to high temperatures such as those found in dryer cylinders.
A departure from the still prevalent concept of spoiler bars is disclosed in published United Kingdom Patent Application 2,032,058 (1980), naming Guntram Feurstein as the originator. The Feurstein published application discloses the use of a wire mesh network or a network formed of "bent strips" for agitating the condensed steam in a dryer, which network is spaced away along much of its length from the inner surface of the drying cylinder to promote flows both over and under the network. As a result, the condensate layer must reach a minimum depth, dependent on the degree of spacing, before any effective agitation is accomplished. In addition, the Feurstein network is attached by fastener screws to the inner surface of the dryer at various points with the attendant shortcomings, i.e. a weakening of the shell, as described above.